The expression “digital position transmitter” used in this disclosure relates to a system which controls one or more output signals corresponding to a plurality of input signals. Some of the input signals represent a static or dynamic set state while others of the input signals characterize a static or dynamic actual state. The output signals are used to make the actual state match the set state. The algorithm for this purpose is implemented in software in a microcontroller. In general, the output signals—with or without the use of auxiliary power—control the position of an actuating element.
(Digital) position transmitters such as these have a position feedback sensor system which detects movements and positions of the connected actuating element. The position feedback of the known (digital) position transmitter is designed such that a shaft transmits the movements and positions of the actuating element to a rotary measurement system, for example a potentiometer. Depending on the technology that is used, the measurement range of the rotary measurement system is restricted to an angle of <120° or <270°.
Actuating drives are divided into pivoting drives and linear drives. In the case of a linear drive, the linear movement of the output drive of the actuating drive is transmitted directly to a linearly operated actuating member. In this case, a pick-up lever converts the linear movement of the push rod of the actuating element to a rotation which is transmitted directly to the shaft. The shaft has a flat on one side. The shaft holder in the pick-up lever is designed such that the pick-up lever is mounted on the shaft in an unambiguous interlocking manner. The design of the mechanical fitting of the digital position transmitter to the actuating element ensures that the position transmitter is mounted at an angle of 90°, transversely with respect to the push rod.
In contrast, in the case of a pivoting drive, the linear movement of the output drive of the actuating drive is converted by suitable means to a rotary movement. In this case, an adapter connects the shaft directly to the pivoting drive.
These known arrangements are standardized by the Standards DIN/IEC 534 and VDI/VDE 3845.
In the known position feedback sensor system with a rotary measurement system for a linear drive, the measured value that is fed back is a trigonometric function of the position in the control range, whose parameters depend on the initial value of the position on set up and the effective lever arm lengths on the push rod of the actuating element and of the pick-up lever. For a positioning behavior which is as accurate as possible, the measured value which is fed back is linearized corresponding to the actual position of the actuating element in the control range. To this end, the software of the digital position transmitter implements a linearization algorithm. In this case, it is necessary to know the position in which the pick-up lever is at right angles to the push rod of the drive.
In order to ensure the commercially required linearity of the positioning behavior, the described design of the position feedback requires the digital position transmitter to linearize the detected pivoting movement in the case of linear drives.
It is known for the digital pneumatic position transmitter to implement a function which specifically moves the connected actuating element by means of self-generated output signals, and thus carries out auto-calibration. In this case, auto-calibration means a function in which the digital position transmitter automatically sets the upper and lower limits of the operating range of the actuating element.
The fitting mechanism and the costs of appropriately high-resolution measurement technology have led to the pivoting range for fitting of linear drives being restricted to an angle of 30° to 60°. Because of the restricted measurement range of the rotary measurement system the pivoting range of the shaft for digital position transmitters is restricted to a defined circular segment relative to the housing geometry of the digital position transmitter. This leads to errors in fitting the digital position transmitter to the drive and actuating element when the resultant pivoting range leaves the permissible circular segment. This problem applies to pivoting and linear drives.
Furthermore, it is known for digital position transmitters to be equipped with a slipping clutch between the measurement system and the shaft. The pivoting range angle is admittedly still limited, but the pivoting range is not defined with respect to the housing geometry. In order to ensure that the digital position transmitter still achieves the linearity required commercially for linear drives, the position of the shaft at which the pick-up lever is at right angles to the push rod must however in this case be “signaled” in some suitable manner to the digital position transmitter.